A conventional sensor, such as a Pirani gauge, is calibrated against several known pressures to determine a relationship between ambient pressure and power dissipated by the sensor. On one hand, since a Pirani gauge may be designed to have a wide dynamic range and be relatively simple and inexpensive, there is a need to be able to use this type of pressure gauge as a substitute for much higher priced gauges such as capacitance manometers and ionization gauges. On a second hand, the growing markets of high-performance wafer-level micro-packaging require miniaturization of the pressure sensors and integration to standard CMOS processes and MEMS micro-devices such as bolometers, gyroscopes and accelerometers. In addition, these markets may require the capability of the sensor for measuring both the gas composition and the gas pressure.
Pressure sensors based on MEMS micro-sensor devices can achieve both low-cost and CMOS-compatible integration. However, existing MEMS micro-sensor devices may show inaccurate pressure readings when subjected to temperature fluctuations, especially at low pressures. US 2007/0069133 A1 relates to such temperature-sensitive micro-bolometer sensor, whereas the electrical response of the sensor depends on the substrate and environment temperatures by virtue of the temperature dependency of the variable resistance. Furthermore, the method for pressure measurement relies on an absolute voltage reading instead of differential voltage readings, wherein absolute voltage reading may be subjected to inaccuracies if fabrication errors occur from one sensor to the other. The method described in this prior art does not provide gas composition measurements.
Pressure micro-sensors showing relative immunity to temperature fluctuations have been developed. U.S. Pat. Nos. 6,945,119 B2 and 7,331,237 B2 relate to pressure sensors integrated in a CMOS circuit. Temperature fluctuation compensation is achieved using a combination of temperature-sensitive and temperature-insensitive elements coupled to a bridge readout circuit. However, temperature compensation of the gauges is achieved at the cost of structural complexity of the device and readout circuit, which impede integration of such sensors into MEMS micro-devices. Furthermore, the method for pressure measurement relies on an absolute voltage reading instead of differential voltage readings which lead to inaccuracies if fabrication errors occur from one gauge to the other. The methods described in these prior arts do not provide gas composition measurements.
An apparatus and method for measuring both the gas composition and the gas pressure is disclosed in U.S. Pat. No. 7,456,633. This disclosure relates to an apparatus comprising a vacuum container, a pressure container, a discharge container, an ion gauge and a residual gas analyzer; arranged in such a way that miniaturization becomes very difficult and integration to standard CMOS processes and to MEMS micro-device is not achievable in practice.
It is therefore the object of the present invention to provide an apparatus and method allowing 1) miniaturization of the wide-dynamic-range sensors to address the high-performance micro-packaging markets and 2) integration to standard CMOS processes and MEMS micro-devices such as bolometers, gyroscopes and accelerometers, 3) relative immunity of the sensor to temperature fluctuations, and 4) capability for real-time gas pressure and/or gas composition measurements.